The reovirus genome consists of 10 segments of double-stranded (ds) RNA. The segments are transcribed by virus-associated RNA polymerase to form capped m RNAs which also function as templates for a putative replicase in virus-infected cells. Each ds RNA segment codes for at least one protein. Two of these, .mu..sub.NS, and .sigma..sub.NS, encoded by genomic segments M3 and S3, respectively, are found only in infected cells. While the function of these two nonstructural proteins is unknown, there is some evidence to suggest that .sigma..sub.NS may act in the selection and condensation of the 10 different single-stranded (ss) RNAs into precursor subviral particles in preparation for ds RNA synthesis.
It is thus known in the art that .sigma..sub.NS has the capacity to bind ss RNAs as part of the putative viral replication system in infected cells. See in this regard Huismans and Joklik, Virology 70, 411-424 (1976). In addition, virus-specific particles sedimenting at 13-19S which are composed solely of .sigma..sub.NS were found to protect 20-40 nucleotide RNA fragments of reovirus mRNAs, including 3'termini, from nuclease digestion. See Stamatos and Gomatos, Proc. Natl. Acad. Sci. U.S.A. 79, 3457-3461 (1982).
The cloning of the reovirus genomic segment encoding .sigma..sub.NS and its sequencing has been described. See Imai et al., Proc. Natl. Acad. Sci. U.S.A. 80, 373-377 (1983) and Richardson and Furuichi, Nuc. Acids Res. 11, 6399-6408 (1983).
However, in the absence of a recombinant DNA system capable of expressing the desired .sigma..sub.NS protein it has not heretofore been possible to produce enough of the protein to fully explore the biological properties of this compound nor was it possible to determine whether the non-specific binding property of (ss) RNAs was retained by a recombinantly produced analog of this protein.